Many vehicles today, such as automobiles, motorcycles, trucks, trains, and aircraft, employ a caliper-disc brake assembly as the primary mechanism for deceleration. The caliper-disc brake system (FIG. 1, prior art) generally includes a pair of brake pads 30 arranged on opposite sides of the annular surfaces of a rotating disc, or rotor 20 coupled to a wheel or other rotating structure. The rotor is disposed between a brake actuating mechanism, of which the brake actuating mechanism is provided outside of the pair of brake pads 30. The brake actuating mechanism generally includes a caliper 26 with at least one pair of opposing pistons 24 which respectively abut the back plates 34 of a pair of brake pads, thus, the brake pads are positioned such that the friction linings 32 face the surfaces of the rotor 20. Alternatively, it is known in the art that a floating caliper design may also be used which may have one or more pistons on one side of the caliper, but no pistons on the opposite side, and it is also known in the art that the caliper may contain more than one brake pad on each side of the disc.
As the wheel (not shown) rotates, the rotor attached to the wheel via a spindle rotates with it along a rotational axis. Alternatively, it is also known in the art that the rotor may be mounted to the drive shaft. When the brake system is actuated, the pistons are brought closer to each other by e.g., a hydraulic pressure mechanism using brake caliper fluid, and the piston and caliper force the brake pads toward the rotor such that the friction linings are forced against the annular surfaces of the rotor from opposing sides. It is also known in the art that brake actuation may be achieved pneumatically, electrically, and/or mechanically. Since the caliper can be attached to the suspension of the automobile (or other vehicle, structure, etc.), it prevents the pistons and brake pads from rotating with the rotor, thus producing a braking force between the brake pad and the rotor.
FIG. 3 illustrates example forces associated with brake actuation. Typically, the piston has an annular face that contacts the brake pad back plate 34 when the brake is actuated, which introduces a compressive normal force (Fpiston) that forces the piston to push the brake pad against the rotor. A bending force (Fflex) may also be produced if the piston contacts only a relatively small area in the center of the back plate. When brake actuation occurs during rotation of the brake rotor, the friction linings of the brake pads are forced against the rotor surfaces and produce a frictional stopping force (Ffriction). Since the brake pads are contained in the caliper and are prevented from rotating with the disc, a shear force (Fshear) that counteracts the friction force (Ffriction) is created within the brake pad. Therefore, the brake pad back plate should be able to withstand the compressive and bending forces encountered during brake actuation, and the brake pad assembly should be able to withstand the shear forces encountered during braking.
Under dynamic braking conditions, friction converts the kinetic energy of the vehicle into thermal energy. Therefore, as the mass and velocity of the vehicle increases, the amount of kinetic energy increases and the corresponding thermal energy produced under braking also increases. The thermal energy produced during frictional braking is transferred as heat into the disc brake components (FIG. 1). Depending on the rate of heat introduced into the braking system compared to the rate of heat expelled from the system, the temperatures of disc brake components may be elevated to a level surpassing their designed operational limits, thus leading to failure. In some high-energy (e.g., racing) applications the temperature of the rotor may be in excess of 700° C. (1,292° F.), the brake pads in excess of 500° C. (932° F.), and the caliper fluid in excess of 200° C. (392° F.). Typical modes of failure may include rotor fracture, loss of frictional force from the friction lining (brake fade), shearing through the friction lining and separation from the back plate, and/or caliper brake fluid boiling (brake fluid fade).
In one example, brake fluid fade occurs when the temperature of the brake fluid (e.g., caliper fluid) reaches its boiling point. In a typical disc brake, the caliper uses hydraulic pressure to generate a force to the pistons that contact the brake pads. The brake fluid in a hydraulic caliper is able to transmit force across a distance because it is able to be pressurized, which is easier to accomplish in its natural fluid state than after it has boiled into a gaseous form. Thus, if the brake fluid temperature reaches its boiling point and vaporizes during use, the result is a partial or complete loss in the ability to transmit force to the brake pad which can result in a loss of deceleration capability. Heat transfer to the caliper brake fluid is primarily achieved by conduction of heat through the brake pad to the pistons 24, and then to the caliper brake fluid. Therefore, the through-plane 42 (z-direction, see FIG. 2) thermal conductivities of the component parts of a brake pad can influence caliper brake fluid temperature.
The overall mass of the brake system may also have an impact on vehicle dynamics such as handling and ride quality. The brake system components (FIG. 1), along with the wheels, tires, and some suspension components are considered unsprung weight (i.e., undamped weight), whereas the remaining weight of the vehicle that is supported by the suspension is considered sprung weight (i.e., damped weight). The unsprung weight of a vehicle is one of the most critical factors affecting a vehicle's road holding ability. Since unsprung weight is not supported by the suspension, it is the weight that is most susceptible to forces induced by bumps and surface imperfections in the road. Therefore, reducing unsprung weight can reduce, such as minimize, the burden placed on controlling the motion of the wheels and tires, which may allow the use of smaller suspension springs and shocks, or may allow the original suspension springs and shocks to have a greater reserved capacity to control vehicle body motion. Since unsprung weight is largely a function of the mass of a vehicle's braking components, reducing the weight of brake components (e.g., brake pads) will generally improve handling and ride quality.
Conventional brake pads typically employ a friction lining that is mechanically fastened, adhesive bonded, or molded to a metal back plate. The friction lining is known in the art and may be comprised of a resin-bonded composite containing resin, fibers, and filler material; or the friction lining may be comprised of a sintered-metallic composite containing sintered metals and filler material; or the friction lining may be comprised of a carbon/carbon composite containing carbon fibers and/or filler material reinforced in a carbon matrix. The types and amount of the constituents in the friction lining are chosen so as to impart the desirable characteristic of the brake pad, including high or low coefficient of friction, high temperature stability, and wear resistance.
The brake pad back plate is typically made of steel, but in some applications may be made of other metals or metal alloys. Metal back plates are typically denser than the friction lining and may therefore contribute a majority of the mass of the brake pad. Also, metal back plates are isotropic materials, such that their thermal and mechanical properties are the same in all crystallographic directions. The isotropic behavior of such metal back plates means that thermal conductivity is the same in all directions (FIG. 4), and therefore heat will generally not be preferentially conducted away from the area of the back plate where the caliper pistons come into contact. Since the thermal conductivity of metal back plates (see Table 1) is typically higher than that of the friction lining, and because metal back plates exhibit isotropic behavior, there is a likelihood for heat to be conducted in the through-plane 42 (i.e., z-axis) direction to the caliper brake fluid via the pistons, which may cause a loss in deceleration ability due to brake fluid fade.
TABLE 1Thermal Conductivity (W/m-K)RatioDensityManufacturerGradeFiber TypeIn plane (X-Y)Through-Plan (Z)(X-Y:Z)(g/cc)Various1010 Steeln/a55.5(a)55.5(a) @ 200 C.1.07.86Various4140 Steeln/a42.3(a)42.3(a) @ 200 C.1.07.83VariousTitaniumn/a17(a)    17(a) @ 20 C.1.04.5Blackhawk31CLChopped 3.2(b) 1.4(b)2.31.6Blackhawk31HDChopped123(b)    46(b)2.71.75Carbon Composites Inc.CCP130-12Fabric lay-up30(c.)    6(c.)5.01.35GoodFellowC413050Fabric lay-up250(d)    50(d)5.01.3(a)ASM Metals Handbook Desk Edition, 1985 pg (1-48, 1-63)(b)Online product data sheet available at www.wellmanproducts.com(c)Online Product data sheet available at www.carboncompositesinc.com(d)Online product data sheet available at www.goodfellow.comRegarding the product data sheet available at www.wellmanproducts.com, 31CL is represented by As Carbonized; 31HD is represented by Heat Treated/Densified.
For the foregoing reasons, it can be beneficial to provide a brake pad assembly that will reduce the mass of the brake pad and in turn reduce the unsprung weight of the vehicle, which will improve ride quality and operational stability. Further, it can be beneficial to provide a brake pad assembly that can resist thermal degradation at the elevated temperatures encountered during vehicle operation and can improve the thermal management of the braking system, whereby improving thermal management means to preferentially conduct heat away from the caliper pistons by increasing the ratio of in-plane 40 (i.e., x & y-axes) thermal conductivity to through-plane 42 (i.e., z-axis) thermal conductivity. Diverting heat away from the caliper pistons will reduce the propensity for losing caliper brake pressure by brake fluid boiling. Additionally it can be beneficial to provide a brake pad assembly that can utilize the aforementioned necessary improvements while still maintaining the ability to use conventional resin-bonded, sintered-metallic, or carbon/carbon friction linings.